1. Field of the Invention
The invention relates generally to the field of measurement while drilling systems. More specifically, the invention relates to methods for reducing the effects of noise caused by “mud” pumps on the signal channel for measurement while drilling systems that use mud flow modulation telemetry or an electromagnetic telemetry.
2. Background Art
Measurement while drilling (“MWD”) systems and methods generally include sensors disposed in or on components that are configured to be coupled into a “drill string.” A drill string is a pipe or conduit that is used to rotate a drill bit for drilling through subsurface rock formations to create a wellbore therethrough. A typical drill string is assembled by threadedly coupling end to end a plurality of individual segments (“joints”) of drill pipe. The drill string is suspended at the Earth's surface by a hoisting unit known as a “drilling rig.” The rig typically includes equipment that can rotate the drill string, or the drill string may include therein a motor that is operated by the flow of drilling fluid (“drilling mud”) through an interior passage in the drill string. During drilling a wellbore, some of the axial load of the drill string to the drill bit located at the bottom of the drill string. The equipment to rotate the drill string is operated and the combined action of axial force and rotation causes the drill bit to drill through the subsurface rock formations.
The drilling fluid (hereinafter “mud”) is pumped through the interior of the drill string by various types of pumps disposed on or proximate the drilling rig. The mud exits the drill string through nozzles or courses on the bit, and performs several functions in the process. One is to cool and lubricate the drill bit. Another is to provide hydrostatic pressure to prevent fluid disposed in the pore spaces of porous rock formations from entering the wellbore, and to maintain the mechanical integrity of the wellbore. The mud also lifts the drill cuttings created by the bit to the surface for treatment and disposal.
In addition to the above mentioned sensors, the typical MWD system includes a data processor for converting signals from the sensors into a telemetry format for transmission of selected ones of the signals to the surface. In the present context, it is known in the art to distinguish the types of sensors used in a drill string between those used to make measurements related to the geodetic trajectory of the wellbore and certain drilling mechanical parameters as “measurement while drilling” sensors, while other sensors, used to make measurements of one or more petrophysical parameters of the rock formations surrounding the wellbore are frequently referred to as “logging while drilling” (“LWD”) sensors. For purposes of the description of the present invention, the term MWD or “measurement while drilling” is intended to include both of the foregoing general classifications of sensors and systems including the foregoing, and it is expressly within the scope of the present invention to communicate any measurement whatsoever from a component in drill string to the surface using the method to be described and claimed herein below.
Communicating measurements made by one or more sensors in the MWD system is typically performed by the above mentioned data processor converting selected signals into a telemetry format that is applied to a valve or valve assembly disposed within a drill string component such that operation of the valve modulates the flow of drilling mud through the drill string. Modulation of the flow of drilling mud creates pressure variations in the drilling mud that are detectable at the Earth's surface using a pressure sensor (transducer) arranged to measure pressure of the drilling mud as it is pumped into the drill string. Forms of mud flow modulation known in the art include “negative pulse” in which operation of the valve momentarily bypasses mud flow from the interior of the drill string to the annular space between the wellbore and the drill string; “positive pulse” in which operation of the valve momentarily reduces the cross-sectional area of the valve so as to increase the mud pressure, and “mud siren”, in which a rotary valve creates standing pressure waves in the drilling mud that may be converted to digital bits by appropriate phasing of the standing waves.
Irrespective of the type of mud flow modulation telemetry used, detection of the telemetry signal at the Earth's surface may be difficult because of two principal reasons. First, while drilling mud as a liquid is relatively incompressible, it does have non-zero compressibility. Consequently, as the pressure variation travels from the valve to the surface, some of the energy therein is dissipated by compression and rarefaction of the mud as the wave traverses the drill string. Second, and more importantly, the pumps used to move the drilling mud through the drill string are very large and powerful, and frequently are of the positive displacement type. As a result, the mud pumps themselves generate large pressure variations in the mud as it is pumped through the drill string, thus masking the pressure variation signal being transmitted by the MWD instrument.
U.S. Pat. No. 6,741,185 issued to Pengyu et al. describes a method exploiting the raw pressure to estimate the parameters of the noise. The estimation is carried out in two separated tasks: the estimation of the instantaneous frequency on one side, and the estimation of other parameters on the other side via an adaptive filtering approach. U.S. Patent Application Publication No. 200710192031 submitted by Jiang Li et al. describes a similar approach using a LMS algorithm to estimate the parameters of the noise. Because both estimators are completely separated, the ability of the foregoing methods to cancel mud pump noise over a broad frequency band is limited. U.S. Pat. No. 4,642,800 issued to Umeda et al. describes a mud pump noise canceling method based on the use of a set of “stroke counters” (devices which count the operating cycles of each cylinder of the pump) to estimate the instantaneous frequency of the mud pumps. However, the estimation of the instantaneous frequency is assumed to vary linearly with the stroke counter output which is not necessarily a valid assumption.
Selected telemetry signals are alternatively provided to an antenna disposed in the drill string that broadcasts low frequency (generally up to about 25 Hz) signals through the formation where they may be detected by a surface antenna such as spaced apart electrodes (hereinafter referred to as “stakes”) disposed in the ground. Examples of electromagnetic telemetry systems are disclosed in U.S. Pat. Nos. 5,642,051, 5,396,232, and U.S. application Ser. No. 11/308,026, each of which are assigned to the present assignee.
The electromagnetic telemetry signal may likewise be masked by signal noise arising from mud pump operation. The mud pumps may create either cyclical electrical interference that mimics the repetitive activity of the mud pumps, or asynchronous noise arising from, for example, electrical interference generated by power drains caused by any sort of mechanical problem.
What is needed is more reliable methods for estimating and reducing mud pump noise for use with mud pulse telemetry and electromagnetic telemetry MWD systems.